Kings
The Kings owned all the land. He made grants of land to his supporters (lords/nobles). The lords had to swear oaths of loyalty to their king.
Lords/Nobles
Once the King had divided his kingdom into specific sections knights or lords would take over those pieces of land. They would offer protection to the king in return for the land and resources.
Villeins/Knights
Once the Lords and Nobles had their pieces of land, they needed to hold up their end of the bargain. This is where the knights come in. The lords and nobles would hire the men of the land to form a small army that defended them as well as the king.
Peasants/Serfs
The serfs had no land of their own. They were hired to work the land for the nobles and lords. They were in charge of farming the land. The serf was bound to the land. If the noble sold the land the serf went with it. This was not much better than being a slave.